The present invention relates to an exothermic instrument for firing an explosive used to fire a squib loaded, for example, with powder and a circuit board used for such an exothermic instrument as well as to a manufacturing method of the circuit board.
Some gas generators have been used as a driving source for such devices requiring large instantaneous motive power as air bags and seat belts for automobiles. The gas generator has a squib and gas generating agent, for example, the mixture of sodium azide and metal oxide with nitrate or perchlorate and reducing agent. The squib incorporates an exothermic instrument comprising an exothermic resistance and electrode pins communicating therewith, and an explosive. When an electric current is made to flow into the exothermic resistance from the electrode pins in the squib, the exothermic resistance generates heat, activating the squib to ignite the explosive. The combustion of the explosive in the squib thus induces the activation of the gas generating agent, which generates a large amount of gas within an extremely short time to inflate an air bag. The gas generating power also drives instantaneously any device that is linked thereto.
Japanese Patent Provisional Publication No. 5-133699 discloses a conventional squib. The squib connects a pair of electrodes formed on a surface of an insulating sheet with an exothermic resistance made of a metal film. The exothermic resistance of the metal film is made to contact directly the ceramic substrate of the insulating sheet.